\documentclass{article}
\usepackage[a4paper, left=1in, right=1in, top=1in, bottom=1in]{geometry}
\usepackage{xeCJK} % CJK support
\usepackage{listings}
\usepackage{amsmath}

\setmainfont{SimSun}                 % main font
\setCJKmainfont[BoldFont=SimHei]{SimSun} % bold fallback

\author{ice}
\date{2019.01.28}
\title{Latex入门学习}

\begin{document}
\maketitle
\tableofcontents

\begin{abstract}
This is abstract.
\end{abstract}

\section{asf}
你好。

\section{as}
hello world

$$ \boxed{\hat{\theta} = \frac{y}{\lambda}x} $$

\[
\hat{\theta} = \frac{y}{\lambda} x
\]
$$
\displaystyle \hat{\theta}̂  = \frac{y\,x}{\|x\|^2}
$$

\end{document}
